reactとは何ですか?To act in response to or under the influence of a stimulus or prompting: reacted strongly to the sarcastic tone of the memorandum. To act in opposition to a former condition or act: composers who reacted against romanticism. To act reciprocally or in return.Chemistry To undergo a reaction: Methane reacts with hydroxyl to produce formaldehyde. Chemistry To cause (a substance or substances) to undergo a reaction.